There are a number of situations where two or more liquids or particles and liquids must be mixed to a high degree of homogeneity, such as mixing of pigments in paint. There are also situations in which a suspended solid must be mixed to a high degree of homogeneity within a liquid, such as a mixture of sand slurry within paint for use on cinder blocks. A common operation to effect this homogeneity is to immerse into the liquid an impeller fixed to a rotatable shaft, as disclosed in Trowbridge et al U.S. Pat. No. 4,171,166, granted Oct. 16, 1979. The impeller should be of such a shape as to create turbulent flow of the liquid when the impeller is rotated by the shaft. Turbulent flow of a liquid within a container includes two types of fluid motion: large-scale (bulk circulation) and small-scale (turbulent eddies). Bulk circulation results when the fluid stream is discharged by the impeller. Turbulent eddies are generated mostly by the velocity discontinuities adjacent to the stream of fluid flowing from the impeller, and are carried to all parts of the container. This turbulent flow effects mixing of the liquids, or of the solid with the liquid.
A typical prior art impeller is shown in FIGS. 8 and 9. The impeller 80 generally comprises a flat metal disk 82 having a plurality of vanes or blades 84 extending from their edges. Often, these vanes are bent slightly upward or downward relative to the plane of the disk, so that fluid flowing along the surface of the disk will be guided below or above the disk, causing a vertical flow within the container.
A common disadvantage of this type of impeller is severe abrasion. The speed with which the impeller wears down, especially if used to mix liquids having abrasive suspended solids therein, is excessive. Over a short time, the impeller will erode and lose its original shape to the extent that it will cease to create the desired currents in the liquid. An expense is incurred every time the impeller needs to be replaced. Replacement of each impeller causes a certain amount of down-time for the task at hand. Further, particles of metal may be broken off the impeller and become suspended within the liquid that is being mixed, contaminating the mixture.
It is an object of the present invention to provide an improved impeller for mixing liquids, which lasts considerably longer than impellers of the prior art.